Use of multi reactor polymerization systems is common in the preparation of polyethylenes. Dual reactor systems for example, are ideal for the formation of in situ polymer blends while providing a large degree of control over the final polymer architecture. By altering the conditions and/or catalysts used in each polymerization reactor, a large range of polymer architectures can be accessed including bimodal high density compositions for pipe having a comonomer content which increases as molecular weight increases, and narrow molecular weight medium density compositions with controlled comonomer incorporation for balanced film properties.
There are several types of multi or dual reactor systems employed for olefin polymerization. These include dual slurry phase loop reactor systems, cascading slurry phase stirred tank reactor systems, in-series dual gas phase fluidized bed reactor systems, in-series solution phase stirred tank reactor systems, in-series solution phase loop reactors, and hybrid systems comprising a gas phase fluidized bed reactor downstream of a slurry phase reactor.
The use of a dual gas phase reactor system is described in U.S. Pat. No. 6,194,520. Ethylene polymer compositions suitable for blow molding applications are produced by i) feeding catalyst, cocatalyst and monomer to a first fluidized bed reactor to form a first polymer component and ii) feeding the first polymer component to a second fluidized bed reactor (which is connected in series) in which a second polymer component is formed. Additional cocatalyst and monomer are fed to the second fluidized bed reactor. The catalysts employed in this dual fluidized bed reactor system were Ziegler-Natta type catalysts.
A multi-stage slurry polymerization process is described in U.S. Pat. No. 5,189,106. In the process, a Ziegler-Natta catalyst is fed to a first stirred tank slurry phase polymerization reactor along with monomer, cocatalyst and hydrogen. A polymer product forms as a slurry in an inert diluent in the first reactor and is then passed via a feed line to a second stirred tank slurry phase polymerization reactor where it is combined with additional monomer to continue the polymerization reaction. By changing the conditions used in each reactor, a polymer composition having two distinct polymer components is formed.
A dual slurry loop process in which a Zeigler-Natta catalyst is used to polymerize ethylene in the presence and absence of comonomer over two slurry loop reactors, connected in series, is described in U.S. Pat. No. 6,225,421.
A hybrid process comprising a slurry loop reactor upstream of a fluidized bed gas phase reactor is discussed in U.S. Pat. No. 5,326,835. In the process, ethylene is first polymerized in an inert low boiling hydrocarbon medium in a slurry loop reactor. Next, the product mixture is discharged and most of the hydrocarbon medium is removed. The resulting polymer product is then transferred to a fluidized bed gas phase reactor where polymerization is completed in the presence of further ethylene and optional comonomer and hydrogen. A Ziegler-Natta catalyst is employed to provide bimodal or broad molecular weight polyethylene compositions.
A dual reactor solution process employing Ziegler-Natta catalysts is described in U.S. Pat. No. 3,914,342. The polymerization takes place in an isooctane solvent in two parallel reactors, each operating at a temperature of 150° C. After a certain catalyst residence time, the polymer solutions formed in each reactor are combined in a mixing zone. After solvent removal, a two component polymer composition is obtained.
U.S. Pat. No. 6,946,521 describes a dual slurry loop reactor process in which a bridged metallocene catalyst is used in each of two reactors which are connected in series. Compositions suitable for pipe applications are generated. The metallocene catalysts are supported on silica and methylaluminoxane is used as a cocatalyst. In this dual loop slurry process, catalyst is fed only to the first reactor.
U.S. Pat. Nos. 5,844,045 and 5,869,575 discuss a dual reactor solution polymerization process in which a constrained geometry catalyst is used in a first continuous stirred tank reactor, while a Ziegler-Natta catalyst is employed in a second stirred tank reactor and which receives an effluent stream (i.e. a polymer solution) from the first reactor. The examples model such a system using a single solution phase reactor in which a first step is conducted in the presence of a constrained geometry catalyst and a second step is carried out in the presence of a Ziegler-Natta catalyst. A narrow molecular weight polymer component having a narrow composition distribution is made in the first polymerization step (or reactor) and a broad molecular weight component having a broad composition distribution is made in the second polymerization step (or reactor). In the examples, a molar excess of an organoborane activator is used relative to a constrained geometry catalyst in the first step of the polymerization process.
A dual loop reactor system effective for solution polymerization of ethylene with 1-octene is employed in U.S. Pat. No. 6,469,103. A constrained geometry catalyst is employed in a first loop reactor to provide a lower density, higher molecular weight component. A Ziegler-Natta catalyst is employed in a second loop reactor to provide a lower molecular weight, higher density component.
A dual stirred tank reactor process for the solution polymerization and copolymerization of ethylene using two different catalysts is described in U.S. Pat. No. 6,277,931. A phosphinimine type single site catalyst is used in a first reactor and a Ziegler-Natta catalyst is used in a second reactor. The process allows for architectural control over polyethylene products having a relatively broad molecular weight distribution. In the process, a molar excess of a trityl borate type activator is added relative to a phosphinimine catalyst in a first polymerization reactor. An alkylaluminoxane is also added as a cocatalyst component.
A two step polymerization sequence, in which a metallocene catalyst is employed as the catalyst in both steps is disclosed in U.S. Pat. No. 5,605,969. The activator employed is an alkylaluminoxane compound. There is no mention of the use of organoborane or ionic activator compounds. The polymerization process is exemplified using a single reactor which is operated under a first and then a second set of polymerization conditions. The metallocene catalyst and alkylaluminoxane activator are added only during the first polymerization step.
A two stage polymerization sequence is described in U.S. Pat. No. 6,995,216. Both multi-step single reactor polymerizations and multi-reactor processes were contemplated. Different polyolefin products are made in different stages or reactors by varying monomer composition, hydrogen concentration or both. The process, which employs a supported and bridged indenoindolyl metallocene catalyst, provides polyolefins which have a broad molecular weight distribution, a broad composition distribution, or both. The patent contemplates the addition of a supported pre-activated catalyst only to a first polymerization reactor when such a reactor is connected in series with a second polymerization reactor (or the addition of a supported pre-activated catalyst only during a first polymerization stage, when two polymerization stages are carried out in a single reactor).
U.S. Pat. No. 6,319,998 teaches the use of two continuous flow, stirred tank, in-series reactors for the polymerization of ethylene with propylene. The ethylene-propylene (EP) elastomeric product is formed in a solution phase in the presence of an activated single site catalyst. By using a two reactor system, a blend is formed in which blend components can differ in their composition, their molecular weight and in their crystallinity. Ethylene-propylene-diene monomer (EPDM) terpolymers may also be formed using the dual reactor process. In the preferred method, solvent and monomers are fed to each reactor, and a single site catalyst is added only to the first reactor. An effluent stream from the first reactor is passed to the second reactor for continued polymerization. The activator used was N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate (DMPFB). Exemplified catalysts included metallocene catalysts and constrained geometry catalysts. The molar ratio of ionic activator (DMPFB) to metallocene catalyst or constrained geometry catalyst was in all cases 1:1, and the catalyst and activator components were fed only to the first polymerization reactor.
The production of EP or EPDM elastomers using a dual reactor solution polymerization process is also described in U.S. Pat. No. 6,545,088. Two continuously stirred tanks reactors are operated in series and polymerization is carried out in the presence of a constrained geometry catalyst, a trispentafluorophenylborane activator, and a methylaluminoxane scavenger. The catalyst, activator and scavenger are fed to each reactor. The molar ratio of the borane activator to the constrained geometry catalyst fed to the first reactor was from 3.4:1 to 5.0:1 (i.e. a molar excess of the borane activator was used in the first reactor). The molar ratio of the borane activator to the constrained geometry catalyst fed to the second reactor was from 3.0:1 to 3.5:1 (i.e. a similar molar excess of the borane activator was used in the second reactor). The patent teaches that first and second reactors may be operated at temperatures of from 65° C. to 90° C. and from 85° C. to 120° C., respectively.
U.S. Pat. No. 6,372,864 describes a dual reactor solution process employing a phosphinimine type single site catalyst in each reactor. In order to activate the catalyst, a different cocatalyst composition is employed in each reactor. In the first reactor, the cocatalyst used contained at least an alkylaluminoxane activator (which is added in a large molar excess relative to the phosphinimine catalyst). In the second reactor the cocatalyst contained at least a trityl borate species (e.g. triphenylmethylium tetrakispentafluorophenylborate, Ph3CB(C6F5)4. Mixtures containing both alkylaluminoxane and trityl borate activators were also contemplated, provided that the ratio of activator components used was different in each of the two reactors. The patent expressly discloses (see Table 1 of U.S. Pat. No. 6,372,864) a dual reactor process in which an excess of methylaluminoxane and a substoichiometric amount of trityl borate is used relative to a phosphinimine catalyst in a first reactor of a dual reactor process. Although favoring methylaluminoxane in a first reactor allowed access to higher molecular weight polymer, the use of substoichiometric amounts of ionic activator also reduced the catalyst activity. This in turn required higher catalyst feed rates to the first reactor (see Table 1 of U.S. Pat. No. 6,372,864) in order to maintain ethylene conversion and polymer split targets over the first and second reactors (i.e. the weight ratio of polymer made in each of the first and second reactors). The high catalyst feed rate required for the poorly active catalyst increased catalyst cost per pound of polyethylene produced and increased the metal residues in the final product. This is not ideal for optimizing process economics and product properties. Thus, an improvement which enhances process operability for the production of high molecular weight polyethylene, without substantially impacting process economics and/or polymer metal residues is needed. Accordingly, we have now discovered that by adding a sufficiently large stoichiometric excess of ionic activator to the second reactor, much of the catalyst activity loss in the first reactor can be reversed in the second reactor and without sacrificing high molecular weight performance. Hence, problems associated with higher product metal residues or accessing improved process operability and high molecular weight polymer in an economical way, are minimized or eliminated. Interestingly, we have also found that polymer made in this way has outstanding optical properties when made into film. Although, use of a small stoichiometric excess of ionic activator, is in fact disclosed in U.S. Pat. No. 6,372,864 (the molar feed ratio to the second reactor of ionic activator to catalyst is 1.2 or 1.3 as shown in Table 1), the disclosed ratios are insufficient to solve the problems highlighted above.